Conventional pulse generators of this type include, besides the pushbutton selector or keyboard, a memory for the temporary storage of the selection signals and a signal converter or coder translating them into decadic pulse trains with the aid of a timer. The memory, the timer and the coder may be combined in a single integrated-circuit chip, allowing the pulse generator to be dimensioned in a compact manner for interchange with a conventional dialing disk. Operating voltage from the line in the open position of the hook switch keeps the pulse generator in a state of readiness and also permits it to transmit incoming call signals to a local ringer, for example. In order to enable the memory to retain the stored data as long as required, the interface unit must also supply operating voltage to the signal generator when the hook switch is closed even though the resistance of that unit, as seen from the central office, should be very low during that phase. Known interface circuits perform only some of these functions so that, for example, two separate circuits must be used for energizing the pulse generator in the open-loop and the closed-loop condition.